Solar light, and in particular ultraviolet radiation (UV), can induce degradation of organic materials such as polymeric films and coatings. Degradation can result in color change as well as deterioration of optical (e.g. formation of haze) and mechanical properties Inhibition of photo-oxidative degradation is important for outdoor applications wherein long term durability is mandatory. The absorption of UV-light by polyethylene terephthalates, for example, starts at around 360 nm, increases markedly below 320 nm, and is very pronounced at below 300 nm. Polyethylene naphthalates strongly absorb UV-light in the 310-370 nm range, with an absorption tail extending to about 410 nm, and with absorption maxima occurring at 352 nm and 337 nm. Chain cleavage occurs in the presence of oxygen, and the predominant photooxidation products are carbon monoxide, carbon dioxide, and carboxylic acids. Besides the direct photolysis of the ester groups, consideration has to be given to oxidation reactions, which likewise form carbon dioxide via peroxide radicals. In addition to degradation caused by ultra-violet light, polymers (e.g., PEN (polyethylenenaphthalate) can degrade from exposure to blue light in the wavelength range of 400 nm to 490 nm.
Thus, industry would find advantage in new methods of protecting substrates from light-induced degradation.